Toners for use in electrophotographic printers include two primary types, namely chemically prepared toners (CPT) and toners made by a mechanical grinding process. CPT has significant advantages over toners made by a mechanical grinding process. In a mechanical grinding process, particle breakage is difficult to control and minimize. Also the shape of mechanically ground particles is more irregular than CPT particles. Hence, the particle size distribution of mechanically ground toner particles is relatively broader than for CPT particles.
There are several types of CPT, depending on the process used to make the CPT. CPT is generally classified as a suspension toner, an emulsion aggregation toner, a dispersion toner, or a chemically milled toner. Of the foregoing, a suspension toner is made by the simplest process. However, the shape of a suspension toner is limited to spherical, and the size distribution of such toner is dependent on how the toner ingredients are dispersed in a monomer used to make the toner. On the other hand, an emulsion aggregation toner involves a more complex process. However, the emulsion aggregation process provides a toner having a relatively narrower size distribution, and the shape and structure of the toner particles are more controllable.
In a typical emulsion aggregation chemically prepared toner process, the toner components include pigment, wax, and a latex binder which are dispersed by use of surfactants. In general, surfactants are typically relatively hydrophilic and consist of low molecular weight molecules or oligomers. These surfactants provide kinetic stability but insufficient thermal stability to the system. A general method of agglomeration of the particles is a method which destabilizes the dispersion, for example, by an acid/base reaction, a cationic/anionic precipitation method, a metal complex additive or by salt precipitation. During the agglomeration process, the surfactant is easily removed from the surface of the particles resulting in particle agglomeration. Hence, the agglomeration process is relatively extreme and fast. However, particle size and particle size distribution of the resulting agglomerated particles are greatly influenced by the stability of the dispersion. Since the surfactant cannot often provide enough stability to the system, excess surfactant is typically added during or after the agglomerating step to provide stability for the newly formed particles. Because there is an excess of surfactant in the system, all of the surfactant is not firmly attached to the agglomerated particles. This excess surfactant is difficult to separate from the product. Unfortunately, excess surfactant results in printing problems such as humidity sensitivity, difficulty in charge control, and other printing problems.
Hence, there continues to be a need for improved chemically prepared toners and to methods which enable easier control of particle size for toners used in electrophotographic printers.